fix: make title and term suggestion UI reactive

[Malayt04]

What?

Closes #579

Fixes two stale editor state issues in the AI experiment UI:

• the title generation toolbar label was not reactive to title changes
• the content classification suggestion button state was not reactive to content changes
• content classification requests could send stale editor content when generating term suggestions

Why?

Both issues came from reading editor state with select() directly in component render paths instead of subscribing reactively with useSelect().

In title generation, the toolbar could keep showing Generate or Regenerate based on the title value from the initial render, even after the title changed.

In content classification, the suggest button could remain disabled or enabled based on stale content, and the generate action could submit older content instead of the latest editor state.

How?

This PR updates the affected components and hook to use reactive editor store reads where UI state depends on live editor data.

• src/experiments/title-generation/components/TitleToolbar.tsx
  • Replaced render-time select() reads for postId and title with useSelect()
• src/experiments/content-classification/components/useContentClassification.ts
  • Replaced render-time select() reads for postId and content with useSelect()
  • Updated the generate handler to read the latest editor content at click time before sending the request
• src/experiments/content-classification/components/SuggestionPanel.tsx
  • Replaced render-time taxonomy lookup with useSelect() for reactive store usage consistency

Use of AI Tools

AI assistance: Yes
Tool(s): Codex
Model(s): GPT-5
Used for: Implementing the component fixes

Testing Instructions

1. Enable the Title Generation experiment.
2. Open the post editor.
3. Focus the title field so the toolbar appears.
4. Start with an empty title and confirm the button shows Generate.
5. Without reloading, add a title and confirm the button updates to Regenerate.
6. Clear the title again and confirm the button updates back to Generate.
7. Enable the Content Classification experiment.
8. Open the Tags or Categories panel in the post editor.
9. Start with content under roughly 150 words and confirm the suggest button is disabled.
10. Without reloading or closing the panel, add enough content to cross the threshold and confirm the suggest button becomes enabled immediately.
11. Remove enough content to go back below the threshold and confirm the button disables again.
12. Add a distinctive final sentence to the content, trigger Suggest Tags or Suggest Categories, and verify the generated request uses the latest editor content.

Screenshots or screencast

Title generation

Before

Title-before.mov

After

Title-after.mov

Content classification suggestions

Before

Suggest-terms-after.mov

After

Suggest-terms-before.mov

Changelog Entry

Fixed - Make title generation and content classification UI react to current editor state.

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Co-authored-by: Malayt04 <malaytiwari@git.wordpress.org>
Co-authored-by: t-hamano <wildworks@git.wordpress.org>

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